Trees in Hardscape: Structural Stability vs Rootable Soil

Soil structure is fundamental for the planting of trees, as well as the stability of roads, buildings, and other infrastructure. It refers to two things:

the arrangement of soil particles including silt, sand, and clay that aggregate together

the void pore spaces between these aggregate particles

In this article, we discuss why appropriate soil structure is critical to healthy tree establishment, and what landscape architects and other specifiers can do to ensure newly planted urban trees receive the conditions they need to thrive, so they can provide the many benefits that mature trees offer.

Why We Need Soil Structure

Not every soil is conducive to the growth of trees and other plants, urban soils least of all. Tree growth and fertility are strongly influenced by soil structure, as it defines whether the tree will have rootable soil available and affects the movement of air, water, and other nutrients required for trees to flourish.

Effectively the ‘architecture of the soil’, soil is usually the most critical element for the success or failure of urban trees. A well structured soil functions like a reservoir, enabling the tree to accept, store, and transmit water and nutrients, provide room in which roots can propagate, and allow the space required for life and the necessary biochemical exchanges for growth.

Trees & Compacted Soil

Too often, trees are planted in cramped planting pits and poor subsoil, resulting in retarded growth, with roots tending to colonize the area immediately underneath paved surfaces, leading to structural pavement damage.

There are different types of roots associated with tree establishment. Anchor roots function as a structural element to the tree and hold the tree in place in the soil.

Fibrous roots (or feeder roots) are roots that intake nutrients, and typically grow in the top 6″ of the soil. They can be directed lower to avoid damage to paved surfaces, if appropriate irrigation and aeration are ensuring the required nutrients are reaching lower levels. This “feeder zone” can extend between two and seven times the diameter of the canopy drip line.

The feeder zone of any tree must be protected from compaction in order to ensure root establishment.

“Resistance to root penetration due to soil compaction will affect root establishment and the ultimate health of the tree“

Tree roots are opportunistic and seek out favorable growing conditions. Moisture and air trapped between impermeable pavements and compacted soil attracts nutrient-starved roots to grow into those areas. The resulting colonization of those roots causes subsequent pavement heave and infrastructure damage.

These trees died prematurely due to unavailable rooting volume caused by soil compaction

So how can urban trees be adequately provided for in their city settings, without compromising or damaging the structural integrity of paved surfaces? The answer is structural soil cells.

How Soil Cells Work

As we discussed in last week’s article, urban soil rarely provides the favorable environment trees need to grow and flourish. Hard compaction, lack of aeration, poor drainage, low nutrient levels, and the existence of pollutants in soil stunt root growth and make it nearly impossible for urban trees to thrive. Landscape architects, arborists, engineers, and other specifiers have the availability of strong soil support systems that, while conducive to root growth, also offer adequate support for roads and sidewalks.

The concept behind soil cells was first developed by GreenBlue in 1992, when our UK branch installed the first tree pit using soil cells to provide uncompacted soil volume for root growth underneath a paved surround.

Suitable for parking lots and sidewalks, soil cells prevent the topsoil in tree pits from becoming compacted by the pressure of surrounding hardscapes, amongst other things. They allow trees in urban settings to have large, healthy root systems, thriving in quality uncompacted soil.

These modular units are assembled into a skeletal framework (or matrix) with over 90% void space to provide large volumes of soil within the tree pit for the healthy growth of roots – all while also supporting pavement loads.

Designed to highly advanced engineering specifications in order to support heavy vertical and lateral loads, StrataCells bring tree root systems closer to the pavement surface. Engineers have calculated that with only 12″ (300mm) of granular pavement depth, a StrataCell matrix can support maximum traffic loads, while providing over 94% of void space for root growth.

With both vertical and lateral forces considered in the engineering make-up of tree pits, soil cell modules lock together, forming a monolithic framework with excellent modular strength. Highly secure connectors allow for modules to click together fast and simply, while it’s enormous growth zone allows for plenteous root establishment.

Structural Integrity of Soil Cells

GreenBlue soil cells are crush tested during manufacturing as part of our rigorous quality control standards. Whilst FEA (Finite Element Analysis) computer load testing was also conducted during the initial design stages to project the loading capacity laterally and vertically, physical laboratory tests were then used to clarify the cells actual loading capabilities.

This physical load testing is part of an ongoing development and research program, and is the only true measure of structural integrity. GreenBlue soil cells are made using 100% recycled polypropylene, and have the highest structural integrity of any large soil cell.

Fatigue Testing

As manufacturer of the world’s strongest soil cell, GreenBlue has subjected our soil cells to extraordinary laboratory tests, including fatigue testing. In one test, a university applied a load of 8.6 tonnes to a StrataCell® tower 10,000 times. The tower was then crushed to measure whether the ultimate load had been diminished by the cyclic loading. The high strength modules had lost no strength, verifying the design of this remarkable system. In closing, our streets do not have to be a battle between trees and pavement. They can coexist together, if specifiers and professionals consider the site conditions and provide an appropriate soil structure to suit both tree planting and infrastructure construction.

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